In a Fourdriner machine a suspension of fibres or "stock" is discharged through an orifice or "slice" onto a moving endless wire screen or "wire," (the "forming wire"). The greater part of the water associated with the stock as discharged from the slice is drained away through the wire, leaving most of the fibres on the wire in the form of a continuous felted fibre mat or web. Such drainage occurs principally in the vicinity of certain forming wire-supporting means.
In the making of paper on a Fourdrinier machine several problems are current. One of these is the problem of obtaining the necessary drainage of the water from the stock without excessive removal of the fine fibres or "fines" as they are known in the art. Another problem known as "kick-up" that was associated with the use of table rolls was to a degree brought under control with the introduction of drainage foils.
With low speed machines, table rolls provide adequate drainage and sufficient disturbance to aid formation at the wet end of the table. There are two mechanisms for the disturbance. One is the upwash of water into the web at the in-going nip and the second is the acceleration imparted by the rapid changes in wire radius of curvature at the out-going nip. Increasing machine speeds leads eventually to excessive upwash with stock jump and reduced fines retention on the downstream side of the roll. Both mechanisms are important for good formation but when using tablerolls they cannot be easily controlled independently of machine speed or drainage rate.
To overcome the stock-jump instability, stationary foils are used in place of some or all the rolls. They have a more gentle dewatering action and, because of the sharp front edge, do not push any significant amount of water back into the sheet. They pull the wire down under the action of the suction. The wire moves back up after running through the suction region and the resultant forming-wire curvature again causes a disturbance to the sheet. This aids formation but, at high speeds, can again lead to stock-jump. Independent control of drainage rate and disturbance level is still not possible.
Foil-type drainage apparatus for paper-making machines involves one or more drainage or dewatering elements ("foils") disposed one after another in the machine direction in fixed relationship to the Fourdrinier wire and extending across the machine transversely to the direction of wire travel. Depending on the width of the paper being made, the foils can be as long as 30 feet, or more. Examples of two different types of such foils are found, respectively, in U.S. Pat. No. 2,928,465 and 3,323,982. The foils are subject to wear, and for this and other reasons it is desirable that they be exchangeable and hence removably mounted on supports. U.S. Pat. No. 3,713,610 proposes a solution incorporating a dovetail slide on the support and a mating mortise slot in the foil for removal and installation of the foil by sliding it lengthwise, across the direction of wire travel. Other solutions exist, all generally incorporating supporting means extending transversely of the forming wire for supporting a foil, and means in the foil for fixing the foil to the supporting means. Owing to the force exerted on the foil in one direction by the wire passing over its leading or "sealing" section, followed by the force exerted on the foil by suction in the opposite direction in its suction-forming section, there is a tendency for a dewatering foil in use to twist around its mounting or supporting means, and this contributes to creep of the foil and to wear of both the foil and the forming wire, as well as to instability in the dewatering process.
Foils currently in use have a sharp front edge, a flat leading or "land" surface for supporting or bearing the forming wire and a diverging trailing surface with divergence angle between 0.degree. and 5.degree. for draining water from the wire. The front edge meets the oncoming forming wire with an acute angle which, with the sharp front edge, sheers off the majority of the water hanging under or otherwise protruding from the wire. If more than one foil is used, as in a set or group of foils working together, each foil is separately mounted and each has one front edge, one bearing section and one drainage section; and each is subject in use to forces tending to twist it around its supporting means.